This invention relates to cables of the type which have a core formed of one or more conductors surrounded by a dielectric material, such as a foam plastic material, and with a metallic sheath surrounding the core and serving as the outer conductor of the cable. Cables of this type are used in the transmission of RF signals, such as television signals, for example.
It has been previously recognized that for cables of this type it is desirable for several reasons to bond the metallic sheath to the core. Bonding of the sheath to the core prevents the core from pulling back within the sheath, and bonding seals the cable against penetration of moisture. The foam dielectric used in the core of the cable is desirably formed of a nonpolar material, such as polyethylene foam for example, so as to avoid adversely affecting the electrical properties of the cable. However, the nonpolar nature of the dielectric makes it difficult to achieve a good bond between the dielectric and the metal sheath. It has also been previously recognized that a particularly suitable class of adhesive for accomplishing bonding of the metallic sheath to the foam dielectric of the core are polar thermoplastic copolymers of ethylene and an ethylenically unsaturated carboxylic acid, and in particular ethylene acrylic acid copolymers. For example, the use of such adhesives in cables is disclosed in the following patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,315,025 Tomlinson April 18, 1967 3,459,877 Bullock et al August 5, 1969 3,662,090 Grey May 9, 1972 3,681,515 Mildner August 1, 1972 3,795,540 Mildner March 5, 1974 4,010,315 Mildner March 1, 1977 ______________________________________
Because of the thermoplastic nature of the ethylene acrylic acid copolymer adhesive, it is typically applied to the substrate with heating, e.g. by extrusion coating or by application of the adhesive in powdered form accompanied by fusion. The adhesive layer is usually about one to five mils in thickness.
The present invention is based upon the recognition that the polar nature of the ethylene acrylic acid copolymer adhesive adversely affects the electrical characteristics of the cable, and for this reason, as well as for reduction in material cost, it is desirable to make the adhesive layer as thin as possible. However, with the conventional methods of application noted above there is a limitation to how thin the adhesive can be applied. Accordingly, one of the objectives of the present invention is to provide a method for producing cable with a metallic sheath bonded to the core and wherein the adhesive can be applied in a thinner layer than heretofore possible. Still another objective of the invention is to provide a suitable adhesive for bonding the core to the metallic sheath wherein the adhesive can be applied in a thinner layer than heretofore possible. A related objective is to provide an improved cable utilizing such an adhesive.
One of the problems which is encountered in applying thermoplastic ethylene acrylic acid copolymer adhesives by the conventional methods heretofore available is that the heat which is necessary for application of the adhesive adversely affects the thermoplastic foam dielectric of the core and thus makes it difficult or impractical to apply the adhesive directly to the core component. For example, extrusion coating of the adhesive directly onto the core component tends to collapse the foam dielectric and thus impair its electrical characteristics. Consequently, the usual practice is to apply the adhesive coating to the metal foil used for forming the metallic sheath, as will be seen from the patents cited above. However, this approach does not lend itself for application of the adhesive to cables in which the metallic sheath component is of an electrically and mechanically continuous construction and formed from extruded seamless tubing or from longitudinally welded tubing.